This guy has hacked his iPhone so it runs Android. In fact, he can choose to run either the iPhone OS or Android. It only works on the original 2G iPhone and it’s a little buggy, and not recommended that casual users try it, but the future could hold implementations on the 3G and 3Gs. Apple’s secret garden just got further pried open.
As commenter pecan 3.14159265 points out:
Well, now everyone who has a 2G and has to upgrade to a new iPhone to get firmware and hardware updates has a use for their old 2G!
Android running on iPhone! [Linux on the iPhone]